With irons, how you hold the rifle or how you look through the sights can certainly affect zero. This is why, in military courses of fire, we zeroed our own.

A consistent hold with your own rifle will help. The technique I was taught with the M16 / AR15 is to touch the tip of my nose against the charging handle when I achieve cheek weld. My sights are zeroed for me, using that technique. Would they be dead on for somebody with a shorter or longer nose, or differently spaced eyes? Perhaps not.